1. Field of the Invention
The present invention relates to optical filters and, in particular, relates to temperature compensated optical filter assemblies having a thin film interference filter sub-assembly deposited therein.
2. Description of the Related Art
Optical filters are commonly used in science and industry to selectively attenuate optical signals according to frequency. For example, communication systems which transmit multiplexed optical signals having a plurality of frequency components along a single optical fiber often rely on optical filters to demultiplex the transmitted signal. In particular, an optical filter adapted to substantially attenuate or reflect all but a narrow frequency band, commonly known as a band pass filter, allows the corresponding frequency component to be isolated from the transmitted optical signal so that the information carried by the isolated frequency component can be subsequently processed in a non-interfering manner.
A typical optical filter assembly comprises a glass substrate, an interference filter deposited on the glass substrate and a holder that supports the substrate. In particular, the glass substrate structurally supports the interference filter, which comprises a plurality of thin films deposited in a layered manner over a surface of the glass substrate. Furthermore, the holder couples with another surface of the glass substrate, wherein the two surfaces are on opposite sides.
When an input broadband optical signal is directed so as to be incident upon the interference filter, the interference filter selectively attenuates the signal by exploiting the well known principles of reflection, refraction, and interference. In particular, the input signal is initially subdivided into multiple broadband constituents as the signal undergoes reflection and refraction at each layer of the interference filter. Thus, each constituent travels along a unique optical path length, defined hereinbelow as the product of the physical path length times the index of refraction of the refractive medium, so that the frequency components of each constituent undergo frequency dependent phase changes. Furthermore, after traveling through the varying optical path lengths within the refracting medium of the filter, the subdivided constituents that eventually exit the rear layer recombine in an interfering manner to produce a transmitted filtered output signal. Likewise, the light energy that exits the front layer recombines to form a reflected filtered output signal.
Thus, the filtering aspects are determined by the thickness and index of refraction of each of the thin films of the interference filter, and the incident angle of the input signal with respect to the interference filter. Consequently, the interference filter may be adapted to perform virtually any specific filtering operation, such as band pass filtering or band rejection filtering, using appropriately dimensioned thin films having appropriate refractive indices. Moreover, the interference filter may operate both as a reflecting device as well as a transmitting device such that the reflected and transmitted signals are complementary to each other.
However, known optical filter assemblies are often sensitive to a change in temperature. In particular, because variations in temperature alter the properties of the thin films, the indices of refraction and the thicknesses of the thin films typically vary in response to a change in temperature. Furthermore, because the glass substrate and the interference filter usually have different coefficients of thermal expansion, the glass substrate usually exerts a thermal mismatch stress onto the deposited thin films that often causes the thin films to experience further temperature dependent changes in thickness. Thus, because the filtering characteristics depend on the indices of refraction and thicknesses of the thin films, a change in temperature often changes the filtering characteristics of the filter.
Consequently, known filter assemblies having substantial temperature dependencies may limit the performance of optical systems that rely on such devices. In particular, the temperature dependent filtering characteristics of known filter assemblies may limit their ability to consistently transmit one signal having a first frequency range while consistently attenuating or reflecting another signal having a second frequency range. Because these devices are often placed in environments having substantially changing temperature conditions, substantial allowances may be required in the design of optical systems that utilize such devices to compensate for the foregoing temperature dependency.
For example, in the case of the multiplexed fiber optic communications system mentioned above, the required frequency spacing between each of the frequency components of the transmitted signal may need to be relatively large so as to accommodate the temperature dependent spectral performance of the filter assembly. Because the maximum number of simultaneous signals that can be transmitted along a single optical fiber is directly related to the minimum frequency spacing, the temperature dependent filter assembly will likely limit the number of simultaneous signals that can be transmitted through the fiber optic cable.
The typical solution used in the industry to reduce the forgoing problem of temperature dependency is to deposit the interference filter on a compensating glass substrate. In particular, the material of the glass substrate is chosen so that the thermal mismatch stress exerted by the substrate onto the thin films induces the thicknesses of the thin films to change such that the filtering characteristics of the filter have a reduced sensitivity to a change in temperature. However, although this approach can be used to reduce the thermal dependency of the filtering characteristics, substantial thermal dependencies often remain. Furthermore, because the compensating glass substrate is typically formed of relatively expensive glass materials, such optical filters are relatively expensive to produce.
Therefore, from the foregoing, it will be appreciated that there is a need for an optical filter assembly having a spectral response that is less affected by a change in temperature. To this end, there is a need for an optical filter assembly that is able to further reduce thermally induced changes in the optical pathlengths of the filter. Furthermore, there is a need for the device to be constructed in a simple manner so that it can be inexpensively produced. Moreover, there is a need for the device to be formed with a small size so as to be usable in space constrained fiber optic systems.